This invention relates to the production of electrically conductive polymer materials and is particularly concerned with the production of such materials exhibiting improved mechanical properties, processability, and thermal and environmental stability, and with procedure for producing same.
The free-base form of polyaniline is believed to comprise subunits having the formula: ##STR1## where n is between and 0 and 1. The oxidation state of polyaniline referred to as "emeraldine" is believed to have a value of n of about 0.5.
The free-base form of polyaniline is electrically non-conductive. Protonic acid doping of polyaniline by reaction of polyaniline with a protonic acid HX where X is, for example, Cl, to produce electrically conductive polyaniline is known, for example, as disclosed in A. G. MacDiarmid, et al, Mol. Cryst. Liq. Cryst. 121, 173 (1985). Conductive polyaniline of this type has been employed in batteries, as disclosed, for example, in French Pat. No. 1,519,729.
However, a number of difficulties have been encountered with the prior art materials noted above. Thus, the conductive polyaniline acid salts are, with a few exceptions, insoluble in most solvent media. None of the polyanilines can be melted. The emeraldine free-base and the conductive forms thereof noted above tend to form powders on removal of the solvent. With some effort, films can be cast; however, they are quite fragile and brittle, easily crumbling to form a powder. The conductive acid salts lose their conductivity when exposed to liquid water. This loss is due to deprotonation. The conductivity loss is reversible; treatment of the deprotonated material with protic acids restores the conductivity. Further, conductive regions in an insulating matrix tend toward diffusion. For example, if one makes a conductive trace of polyaniline acid salt on a substrate of emeralidine free-base, the trace remains spatially stable for only a short time, eventually spreading out until the substrate has a constant conductivity throughout.
Some of these problems were addressed in U.S. Applications Ser. No. 920,474 filed Oct. 20, 1986, now U.S. Pat. No. 4,798,685, of S. I. Yaniger, and Serial No. 013,305 filed Feb. 11, 1987, now U.S. Pat. No. 4,806,271, of S. I. Yaniger, et al, both assigned to the same assignee as the present application. In these applications, it is disclosed that Lewis acids, for example, alkylating agents, can be used to make the insulating emeraldine free-base into a conductive polymer salt. Use of proper Lewis acids resulted in conductive polyanilines with the Lewis acid as a sidechain. These derivatized polyanilines are more water stable and processable than the prior art emeraldine acid salts. Additionally, no diffusion between "doped" conducting and "undoped" insulating regions was observed.
Thus, in the above U. S. application, Ser. No. 920,474, a base-type non-conductive polymer, such as polyaniline, can be reacted with, for example, methyl iodide, to form an electrically conductive polymer in which the methyl group is covalently linked to the nitrogen atoms of the polymer.
In the above U.S. application, Ser. No. 013,305, emeraldine free-base can be reacted with reagents of the form RSO.sub.2 Cl, e.g., tosyl chloride, to form an electrically conductive polymer in which the --SO.sub.2 R groups are covalently linked to the nitrogen atoms of the polymer.
In general, however, the conductive polymers of the above applications do not form consistently good films. Attempts to cast films by removal of solvent from polymer solutions generally yield powders or extremely brittle, fragile films.
Additionally, polymers of the above applications exhibit loss of conductivity when exposed to high temperatures. For example, the conductive polymer produced by emeraldine free-base treated with tosyl chloride (conductivity =1.0 S/cm) loses its conductivity when heated to 150.degree. C. for one hour. The thermal instability results from interaction of the relatively nucleophilic chloride counterion with the polymer backbone.
An object of the present invention is the provision of improved base-type electrically conducting polymers of the class of conductive polyaniline.
Another object is to provide conductive polymers, such as conductive polyaniline, having thermally stable counterions in the conductive polymer system.
A further object of the invention is the provision of conductive nitrogen-containing polymers having bulky side chains bound to nitrogen atoms of the polymer.
Still another object is the provision of a conductive base-type polymer of the above type, such as conductive polyaniline, having improved mechanical properties and processability.
Yet another object is to provide novel procedure for producing the above conductive polymers.